This invention pertains to rigid disk storage devices and more particularly to a method of allocating data storage locations on the disk surface responsive to the change of lubricant thickness as a result of lube migration over time.
Magnetic disk drives utilize disks manufactured to have a very uniform flat surface with the ultimate coatings being a sputtered magnetic coating and usually a thin protective coating and finally a thin film of lubricant. The protective coating and lubricant are very thin since they occupy a part of the space that separates the magnetic storage layer and the transducer carrying slider that flies only a few microinches above the disk surface. As data storage densities get greater, the number of tracks per inch and the number of bits per inch increase and also the layer of magnetic material becomes thinner. As the magnetic domains become smaller, the slider must fly lower to reduce the separation between the transducer and the magnetic surface. These conditions lead to more frequent incidental contact between slider and disk.
The lubricant may be one or multiple materials, but the overall lube normally has two portions. One portion of the lube bonds chemically to the underlying surface material and the remainder is mobile. The chemically bonded fixed lubricant is only a small portion, even when only one lubricant material is applied. The mobile portion of the lubricant film migrates outward over time, leaving the inner tracks of the band of data storage tracks with little but the bonded lubricant resident at the disk surface. Thus, any intermittent slider disk contact with the data track area near the inner diameter of the data band is much more detrimental to disk life than such disk contact with the data area adjacent the outer radius of the data band. The lube migration does not result in a linear distribution, but causes a greater concentration at the outer periphery of the disks.
The majority of data storage tracks tend to be substantially depleted while only a minority of the total number of tracks at the outermost diameter of the band of data tracks retain any significant thickness of mobile lubricant film.
To accommodate the migration of disk surface lubricant that occurs over time it is prudent to adaptively control the location of data in a manner that causes the frequency of data accesses more closely mimic the lubricant thickness.
The method of the present invention causes the drive control system to periodically review the stored data on the disk surfaces during the idle time between read/write commands. By identifying data files recently accessed and data files most frequently accessed during a predetermined number of most recent operating hours and storing such data files in the outer portion of the storage band while moving other data files to the inner portion of the band of data storage tracks the data accesses occur most frequently in the disk storage area that is more adequately lubricated. This is accomplished by recording, with respect to each stored data file, the data file length, the last access in terms of power on hours (POH) and the number of accesses during a most recent period. Comparisons are made with the current POH value and the most recent period is a fixed moving period of POH that ends with the current value of POH.
Since the lube thickness across the data band is not linear, the preferred location for frequently accessed data includes less than the outer half of the band. Accordingly, in the embodiment of the invention describer hereafter, the data band is arbitrarily separated into the outer third and the inner two thirds. This partitioning may be varied by the migration characteristics of the particular lubricant used, the rotational velocity of the disks, and the location of the disk in the disk pack.